In appliances for tying cable harnesses or other articles by means of a flexible strap which is to be secured in the tensioned tying position by means of a closing device, the limitation of the strap tension to a predetermined value is an important factor, since the article to be tied or the strap itself could be damaged as a result of too high a strap tension. The state of the art uses different forms of so-called load balances for this purpose. Thus, a coupling is inserted into the force transmission path, which transmits the tensioning force via an inclined plane and is then released when the force component acting transversely relative to the normal force transmission path as a result of the inclined plane exceeds a specific force threshold determined by an adjustable spring force. The way in which this principle is put into practice can differ greatly (German Offenlegungsschrift 2,510,575, G.B. preliminary publication 83 25128, WO 82/02867, German Offenlegungsschrift 1,907,306), but at all events involves a very high outlay. However, it has hitherto been considered essential, because the limiting tension must still be adjustable reliably even after many work cycles.
The object on which the invention is based is to put this into effect at a lower outlay.
In the solution according to the invention, the tensioning member acting on the strap is designed to slip past on the strap to be tensioned, when a predetermined strap tension is reached.
This solution is surprising in as much as it amounts to forming a slip coupling between the tensioning member and the strap to be tensioned, but slip couplings are known not, as a rule, to allow an exact and always constant setting of the slip-force limit when there is a large number of work cycles and the constructional size of the coupling has to be kept small in relation to the force to be transmitted. This is because, with slip couplings, it is necessary to allow for wear which so changes the interacting coupling surfaces that there is no permanent guarantee of a calculable functioning.
Furthermore, from a tribological point of view strict requirements are demanded of the properties of the materials forming the slip coupling. However, those materials, of which elastic straps for tying cable harnesses typically consist, are far from meeting such requirements demanded of the material properties, since they are chosen for completely different reasons. It is all the more surprising that the known disadvantages of slip couplings do not arise where the invention is concerned. There are two reasons for this. On the one hand, for each work cycle the coupling is provided with a new friction partner which is in the form of a new strap and the properties of which have not yet changed as a result of preceding work cycles. On the other hand, it is true that an essential wear-related change occurs during the slipping of the coupling past the friction partner foraged by the strap, because the friction partners of the coupling are now pressed together only with a reduced force as a result of the wear. However, this does not have a negative effect because it can easily be ensured that the closing device for the strap is closed immediately as soon as the predetermined strap tension is reached. This is guaranteed at once when a strap with a self-locking closing device (EP-A-35,367, FIG. 2) is used, that is to say one in which the closing position of the strap is determined by the maximum strap tension reached during the tensioning operation, whilst a subsequent reduction of this tension remains without any influence. It is advantageous, furthermore, if the invention makes use of straps made of plastic or at least of a material which is soft in comparison with the material of the other coupling part and which does not impart any appreciable wear to this other part of the coupling.
The tensioning member is appropriately formed by the tensioning roller. The limiting tension at which the tensioning member slips past is determined by the adjustability of the distance between the tensioning member and an abutment supporting the strap on the side facing away from the tensioning member. However, there will also be the possibility, instead, of making the force with which the strap is pressed against the tensioning member by the abutment adjustable. The abutment is appropriately a roller, especially when the tensioning member too is a roller.
Although the surface of the tensioning member can be made smooth, the frictional force acting between this surface and that of the strap being determined by the pressing force, nevertheless, since the random surface state of the strap can lead to differing results, the tensioning member is preferably designed with projections penetrating into the strap surface. When the tensioning member slips past relative to the strap, these projections bring about a deformation of the strap material which can be of a varying kind, depending on the type of projections. If the projections are sharp, a scraping or even chip-removing deformation of the strap can occur. This solution is generally preferred, because the most easily reproducible results are thereby obtained, as long as the strap material remains the same. It is also possible, however, to make the projections obtuse, in which case the plastic displacement of material (in addition to any material abrasion) can be decisive.
Since many plastic straps which can be used for the purposes according to the invention, are toothed on one side, at first sight it seems obvious to cause the tensioning member to act on the toothed side of the strap, in order to utilize for the tensioning operation the positive engagement possible as a result of the toothing. This will, indeed, be possible within the scope of the invention. But, as a rule, the action of the tensioning member on a non-toothed surface of the strap is more advantageous, because it has been shown that more easily reproducible results can then be obtained.